1. Field of the Invention
The present invention relates to a method of controlling an injection molding machine by use of pressures of resin in a nozzle.
2. Description of the Prior Art
Conventionally, in an injection molding machine, a screw feeder which is rotatable and movable forwardly and backwardly is provided in a heating cylinder and actuated by a drive system including an injection cylinder, a hydraulic motor and so forth.
Prior to the injection, resin metering is first conducted, and the screw feeder is retracted while it is rotated. At the same time, resin supplied from a hopper into the heating cylinder is heated by the heating cylinder, moved forwardly in accordance with the retraction of the screw feeder, and stored at the front end of the screw feeder.
Subsequently, when the screw feeder is advanced, the resin stored at the front end of the screw feeder is injected from an injection nozzle and filled in a cavity in a mold. Then, after the injection is completed, the resin in the cavity is cooled down and shaped as a molding product. At the same time, however, the resin is contracted, with cooling thereof.
In consequence, even after the injection is completed, a dwell pressure is applied on the resin in the mold cavity to thereby compensate for the contraction. Then, after the dwell pressure application step is over, the screw feeder is rotated again to start the metering step.
By the way, in the molding cycle comprising the above-described steps, an advancing and retracting speed, a rotational speed and a back pressure of the screw feeder, a hydraulic pressure, and the like are detected, and feedback of these detected values is conducted to thereby control the injection molding machine.
For example, there has conventionally been proposed a method in which a screw feeder is controlled in injection and dwell pressure application steps of injection molding, and a flow and a pressure of the resin are indirectly controlled by regulating pressures and speeds of the screw feeder at the respective positions while it advances (see Japanese Patent Examined Publication No. 57-59060).
Also, there has been proposed a method in which a resin pressure sensor is installed in a passage for resin in a mold, and an amount and a pressure of fluid supplied to an injection cylinder are controlled so that a transition of a detected resin pressure will be the same as a predetermined wave form of a pressure inside the mold (see Japanese Patent Examined Publication No. 58-52486).
Further, there has been proposed a method in which a nozzle valve is provided on the distal end of an injection nozzle, and in an injection step, resin in a heating cylinder is injected when opening the nozzle valve after compressing the resin until the advancing speed of a screw feeder becomes a certain value or less (see Japanese Patent Unexamined Publication No. 2-81617).
However, in the first one of the above-described conventional control methods of injection molding machines, an amount and a pressure of fluid supplied to an injection cylinder are actually controlled. But the screw feeder merely presses the resin. It is the resin that actually moves, and the resin stored at the front end of the screw feeder is injected from an injection nozzle, passed through a sprue, runners, a gate and so forth of a mold, and filled in a cavity. The molten resin is viscous, elastic compressed fluid, and consequently, however accurately the screw feeder is controlled, there are large fluctuations in a density of the resin at the front end of the screw feeder, a back flow amount in a screw feeder head portion, timing of injecting the resin from the tip of the injection nozzle, time for the resin to reach the gate, timing of passing the resin through the gate, time for filling the resin in the cavity, and so forth.
Moreover, in the second control method, since the resin is directly controlled, the control can be performed highly accurately. However, if there is a fluctuation in the density of the resin stored at the front end of an injection nozzle, it affects the control. Besides, since the resin pressure sensor must be installed in the mold, it is difficult to utilize conventional molds in stock at a molding site.
Furthermore, in the third control method, the resin is injected after its density is made uniform by compressing the resin by use of the nozzle valve, and therefore, a favorable molding density can be obtained. However, it is similar to the first control method in that because the screw feeder is an object to be controlled, the control can not be performed highly accurately.